1. Field of the Invention
This invention relates generally to light amplifier tubes and is concerned more particularly with means for enhancing the photon-to-electron conversion efficiency of image intensifier tube input screens.
2. Discussion of the Prior Art
Generally, an image intensifier tube comprises a tubular envelope closed at one end by a radiation transmissive faceplate having an input screen disposed adjacent the inner surface thereof. The input screen may include a scintillator layer of radiation sensitive material aligned with the faceplate and supporting on its inner surface a substantially coextensive layer of photoemissive material. In operation, a radiational image impinging on the scintillator layer causes it to fluoresce locally in accordance with the spatial distribution of photons in the image. As a result, the photoemissive layer emits an equivalent electron image which may be amplified to produce a corresponding bright visible image. Thus, the brightness of the visible image may be enhanced by increasing the photon-be-electron conversion efficiency of the input screen.
One method know in the prior art for improving the conversion efficiency of the input screen includes the processing step of oxidizing the vacuum interface surface of the photoemissive layer to obtain a peak value of conversion efficiency. Accordingly, the tube may be connected to an exhaust system having therein an oxygen source disposed to release a desired quantity of oxygen into the tube envelope after deposition of the input screen photoemissive layer. As a result, the released oxygen migrates to the exposed inner surface of the photoemissive layer and oxidizes the the material thereof. Subsequently, the tube is sealed-off from the exhaust system, and is submitted to final test. During final test, it may be found that the photoemissive layer of the input screen has not been oxidized sufficiently to provide the peak value of conversion efficiency. However, since the tube is sealed-off from the source of oxygen, the material of the photoemissive layer cannot be oxidized to increase the conversion efficiency of the input screen.
Therefore, it is advantageous and desirable to provide method and means for enhancing the photon-to-electron conversion efficiency of an image intensifier tube input screen, even after the tube envelope has been sealed.